Existing techniques whereby a water-in-oil emulsion may be resolved into its constituent phases make use of chemical and physical means for achieving the resolution.
The need for an emulsion breaking operation may arise in connection with a wide variety of processes. For example, in the production of crude oil at the well-head, where saline formation water is present as fine stable droplets in the oil coming to the surface, it is necessary to break the emulsion in order to dehydrate the oil.
In other cases, it may be essential to recover the components of an emulsion after it has been deliberately created for a specific purpose. This occurs in processes using emulsion liquid membrane technology where, for instance, a very stable water-in-oil emulsion is used to treat an aqueous effluent feed stream to remove an impurity. The species to be removed from the aqueous feed stream transfers through the outer oil phase of the emulsion into the water droplets or internal phase of the emulsion. To encourage this mass transfer to occur it is necessary to have some appropriate chemical or physical driving force. A simple example of such a process is the extraction of ammonia from an effluent water into an emulsion of sulphuric acid in kerosene.
A crucial feature of the economics of processes that employ emulsion liquid membranes concerns the ability to re-use the oil phase repeatedly. Ecohomic viability depends on being able to break the used emulsion speedily to remove the spent internal phase in bulk and recycle the oil phase to make fresh emulsion for re-use.
In the oil industry, crude oil emulsions that are slow to resolve using a purely physical method such as gravity separation, can often be treated with chemical additives which serve to destabilise the emulsion. However, the latter course of action is not feasible for liquid membrane emulsions. Here the desire to recreate a stable emulsion using the separated oil phase mitigates against the use of chemical demulsifiers. In the case of liquid membrane emulsions it is therefore necessary to consider the use of physical methods alone to speed up the separation of an emulsion.
In general, the resolution of emulsions requires that the small droplets of the dispersed (internal) phase coalesce together, until they become large enough to be removed easily from the continuous phase. Where the densities of the two phases are different, the denser phase simply gravitates from the emulsion and, given enough time, the two phases can be separated sufficiently for each to be drawn off. The time required for this separation is reflected by the size of the settling tanks which are typically required. These may be very large and may contain a large inventory of expensive liquids. In addition, the phase separation step may be the slowest stage of a more extensive process and therefore limit the throughput of the overall process.
During the resolution of a water-in-oil emulsion, those droplets that have grown by coalescence must gravitate through the emulsion to reach the bulk interface between the separated water layer and the unseparated emulsion. The viscosity of the emulsion tends to be high, especially for liquid membrane emulsions. This hinders the passage of the water droplets and they can be very slow to reach the bulk interface.
One method that has proved useful specifically for enhancing the rate of separation of a water-in-oil emulsion is to subject the emulsion to an applied high voltage gradient. The electric field assists the process of phase separation by promoting coalescence between the water droplets in the emulsion. Several possible mechanisms for electrically aided coalescence have been identified [Waterman, L. C. (1965) Chem. Eng. Progress, vol 61, (10), 51], all of which rely upon the attraction of opposite charges on adjacent droplets to cause an increased incidence of collision followed by coalescence. The larger droplets produced by this process gravitate much more quickly to the bulk interface than their much smaller antecedents could do when under the same gravitational field.
Many different types of electric field have been shown to be effective for emulsion resolution, including: AC fields [Cottrell, F. G. (1911) U.S. Pat. No 987 114, March]; pulsed AC fields [Wolfe, K. M. (1944) U.S. Pat. No. 2,364,118]; DC fields [Siebert, F. M. and Brady, J. D. (1919) U.S. Pat. No. 1,290,369, January]; and pulsed DC fields [Stenzel, R. W. (1958) U.S. Pat. No. 2,855,356, October].
A particular feature of some oil field emulsions and most liquid membrane emulsions is that the water content of the emulsion may be 20% or more. Under such circumstances it is often impossible to apply the desired electric field by simply passing the emulsion between metal electrodes because the presence of a large number of aqueous phase droplets tends to cause an electrical short-circuit between the electrodes. This problem can be overcome by coating the high voltage electrode in a layer of insulation, so that those conduction paths that do develop only cause a localised diminution of the electric field rather than a complete collapse of the field throughout the inter-electrode region.
The potential difference that must be sacrificed when introducing a layer of insulation between the high voltage electrode and the emulsion may be minimised by operating under pulsed voltage conditions. At optimum frequency, pulsed DC fields have been shown to be particularly good for promoting coalescence when the high voltage electrode is coated with insulation [Bailes, P. J. and Larkai, S. K. L. (1986) U.S. Pat. No. 4,601,834, July]. Experimental evidence suggests that best performance is achieved when the emulsion to be treated flows in close proximity to the insulation coated electrode [Bailes, P. J. and Larkai, S. K. L., Proceedings of the International Solvent Extraction Conference 1990 (Kyoto) (Process Metallurgy, Vol 7b, p1411, publd. Elsevier Science 1992)]. It follows from this that the larger the electrode area in contact with the emulsion the better.
The present invention aims to provide a method and apparatus for separating the components of an emulsion, which can allow improved separation rates to be achieved. The invention also provides a novel method of effectively using insulated electrode surfaces. In practice, the new method works in an unexpected way causing another beneficial effect which acts synergistically to bring about a substantial improvement in the rate of emulsion resolution.